Hi, I’ve also tried to access the built-in SC7A20 accelerometer on Arduino Stella via I2C (using the example code from the Grove-3-axis-digital-accelerometer-2g-to-16g-LIS3DHTR library and scanning for 0x18, 0x19, 0x1C) and failed to obtain any values.
After that, I attempted to initialize SPI directly through Stella pins, and it bricked the board completely, probably because some of the pins became messed up. Now, Windows detects a USB device for a second, but immediately shows that it malfunctioned. The Arduino IDE no longer detects the board at all. Pressing RESET or KEY on Stella also does nothing.
I cannot find any documentation about how the IMU is actually connected inside the Stella, and I also cannot find any bootloader files for Stella in the ArduinoCore-renesas repository, so I’m unable to restore the board using an external programmer. Please don't repeat my mistakes 
Here is the code I uploaded last that led to the malfunction:
#include <SPI.h>
// SC7A20 chip select pin
#define SC7A20_CS p41
// Registers
#define CTRL_REG1 0x20
#define CTRL_REG4 0x23
#define OUT_X_L 0x28
SPISettings scSettings(1000000, MSBFIRST, SPI_MODE3);
// SPI: write 1 byte
void sc7a20_write(uint8_t reg, uint8_t value) {
digitalWrite(SC7A20_CS, LOW);
SPI.beginTransaction(scSettings);
SPI.transfer(reg);
SPI.transfer(value);
SPI.endTransaction();
digitalWrite(SC7A20_CS, HIGH);
}
// SPI: read 1 byte
uint8_t sc7a20_read(uint8_t reg) {
digitalWrite(SC7A20_CS, LOW);
SPI.beginTransaction(scSettings);
SPI.transfer(0x80 | reg); // read
uint8_t v = SPI.transfer(0x00);
SPI.endTransaction();
digitalWrite(SC7A20_CS, HIGH);
return v;
}
// SPI: read 6 consecutive bytes
void sc7a20_readBytes(uint8_t reg, uint8_t* buf, uint8_t len) {
digitalWrite(SC7A20_CS, LOW);
SPI.beginTransaction(scSettings);
SPI.transfer(0xC0 | reg); // 0x80 = read, 0x40 = autoincrement
for (int i = 0; i < len; i++)
buf[i] = SPI.transfer(0x00);
SPI.endTransaction();
digitalWrite(SC7A20_CS, HIGH);
}
void setup() {
Serial.begin(115200);
delay(300);
pinMode(SC7A20_CS, OUTPUT);
digitalWrite(SC7A20_CS, HIGH);
SPI.begin();
Serial.println("Initializing SC7A20...");
// Enable X/Y/Z + 100 Hz
sc7a20_write(CTRL_REG1, 0b00110111); // SC7A20: 100Hz, LowPower ON, XYZ enable
// High resolution, ±2g
sc7a20_write(CTRL_REG4, 0b00001000); // High resolution disabled in SC7A20 (works better)
delay(100);
Serial.println("SC7A20 ready!");
}
void loop() {
uint8_t raw[6];
sc7a20_readBytes(OUT_X_L, raw, 6);
int16_t x = (int16_t)(raw[1] << 8 | raw[0]);
int16_t y = (int16_t)(raw[3] << 8 | raw[2]);
int16_t z = (int16_t)(raw[5] << 8 | raw[4]);
// 1 mg/LSB in high-res mode
float xf = x / 1000.0;
float yf = y / 1000.0;
float zf = z / 1000.0;
Serial.print("X: ");
Serial.print(xf, 3);
Serial.print(" g Y: ");
Serial.print(yf, 3);
Serial.print(" g Z: ");
Serial.print(zf, 3);
Serial.println(" g");
delay(100);
}
Any help would be greatly appreciated, thanks!